In certain signal and control systems, such as audio frequency (AF) train detection and cab signal operations, it is mandatory to ensure the safety and security of the crewmen and/or passengers as well as to protect the system and apparatus against damage and/or destruction by a false or unsafe condition. Thus, it is essential to exercise extreme care in designing the circuitry and in selecting the components of the electronic track circuit equipment to ensure fail-safe operation. For example, these precautionary measures should be implemented on each and every portion or circuit which makes up a vital system. In one application, it is required to compare two trains of pulses which emanate from two different supply sources. One such case is the comparison of the outputs of two unijunction oscillators to determine if the oscillators are running in synchronism. As noted above, it is mandatory that the comparison, verification, or coincidence check be carried out in a fail-safe manner.